The present invention generally relates to polyaxial screw technology and, more specifically, load sharing and its application in polyaxial screw technology for the spine. In a preferred embodiment, the technology can be applied to the treatment and correction of scoliosis.
More specifically, polyaxial screw technology has been in existence for a number of years. While the technology has advanced, the focus of key advances have been on providing smaller and stronger means for fixing a screw that fixes a body member and rod assembly to vertebrae, with each screw assembly having the basic structure of a body with pivot means around the screw head and a rod slot. If the rod to be disposed in the rod slot is not centered relative to the screw head, the body can pivot over to adjust for the misalignment. Examples of such systems are abundant in the art.
Once the polyaxial screw is connected to the rod, the assembly is locked such that the screw angulation is fixed relative to the body portion. For example, U.S. Pat. No. 6,740,086 to Richelsoph, issued May 25, 2004, shows one such system.
In a more uncommon approach, U.S. Pat. No. 4,805,602 to Puno, et al., issued Feb. 21, 1989 discloses micromotion between a round screw head and a seat of a body member such that the screw is allowed to rotate in the seat. More specifically, the assembly includes a rod and plurality of vertebral anchors that are positioned on the spine on either side of the spinous process spanning the portion of the spine to be immobilized. The rod is secured to the vertebral laminae by the vertebral anchors. The anchor includes a transpedicular screw member which is secured to a vertebrae. A rod support or body member includes a cup which captures the screw and optionally permits micromotion between the rod support and the screw. This type of approach leads to other issues, as the surgical correction to the spine cannot be effectively controlled and issues of the head rotating in the socket producing failure of the system. Therefore, this type of approach is not common. Of course, the locking strength of the spherical head of the screw in the body portion varies from design to design, as well as the locking mechanism.
A more specific issue related to polyaxial screw technology is the application of load sharing, which has various advantages, including reduction of adjacent segment degeneration and improved fusion quality. The term “load sharing” in spine relates to the ability of a spinal stabilization device to share loads otherwise placed solely upon the spine or soley on the implants. Conventional methods of spinal fixation utilize a relatively rigid spinal fixation device to support an injured spinal segment or segments being surgically corrected. Such fixation limits movement of the injured segment. These conventional spinal fixation devices connect and couple rods or plates to fixing screws such that the injured spinal segment is supported and held in a relatively rigid and fixed position by the rods or plates. The connection units, such as the rods and plates, are usually used during fusion, whereby bone graft is inserted into the space and the implants act as internal braces to stabilize the spine during the bone healing and fusion process. The connection units also reduce pain and further injury to the patient by substantially restraining the movement of the spinal column. However, because the connection interferes with normal movement of the spinal column, negative effects, such as degradation of other healthy segments or pseudoarthrosis can occur causing further complications and issues associated with the spinal column. More specifically, and in the case of large diameter rods, high rigidity of the rods and/or plates used in conventional fixation devices, the patients fixed joints are not allowed to move after the surgical operation. Consequently, such spinal fixation devices cause decreased mobility of the patient and increased stress on the remainder of the spinal column joints adjacent to the operated area. Such excessively rigid spinal fixation can result in what is termed “stress shielding,” whereby the bone graft used for fusion does not receive sufficient loading to allow for solid fusion. By altering this approach and allowing load sharing in the spine, we now have a reduction of adjacent segment degeneration and improved fusion quality.
An early approach for load sharing was a basic system change from a larger diameter rod to a smaller diameter rod. Newer techniques use more flexible rods or complex mechanisms placed as connectors between rod segments.
For example, U.S. Pat. No. 6,241,730 to Alby, issued Jun. 5, 2001, uses a complex link with moveable parts. More specifically, the Alby patent loses an intervertebral link device including at least one damper element constituted by a cage and a pin designed to be connected to bone anchor elements. The pin is engaged in a housing of the cage and is fitted with two elastically deformable members operating in opposition to an applied traction force or compression force. The damper element is a pin that is mounted inside the cage by a joint allowing multi-directional relative pivoting between the pin and the cage, at least about the axis contained in a plane perpendicular to the pin and angular abutment between the cage and the pin enables the multi-directional relative pivoting to be limited in amplitude to a predetermined value of about 4°.
U.S. Pat. No. 7,326,210 to Jahng, et al., issued Feb. 5, 2008, uses a flexible rod constructed from two different materials. More specifically, the flexible connection unit used for use in a spinal fixation device includes a longitudinal member having first and second ends and at least one spacer located between the first neck and second ends wherein the spacer includes a first portion made from a first material and a second portion made from a second material and at least one flexible member located in a longitudinal axial channel of the spacer wherein the first and second ends substantially limit motion of the spacer in the longitudinal axial direction with respect to the flexible member.
Both of the above techniques have drawbacks due to the complexity, size, strength, or inability to integrate into effective spinal stabilization systems. Their use is substantially directed to fusion techniques. Stabilization of the spine for non-fusion is a totally different matter raising totally different issues. Flexion of the spine creates very high loads on the screw-bone interface and often causes loosening of the screws from the vertebrae. Common complications are for the screw to be pulled loose or screw failure, thereby totally destabilizing the fixation device.
The present invention provides a much simpler device from an engineering point of view yet effective in both fixation during fusion and non-fusion techniques. This allows for many options in the treatment of the spine with the same basic system. In addition, the present invention can be utilized for stabilization and reduction during the treatment of scoliosis.
Scoliosis is the medical term for curvature of the spine. Scoliosis occurs in approximately 2% of women and less than ½% of men. It usually starts in the early adolescence and may gradually progress as rapid growth occurs. However, scoliosis can occur at any age from juvenile to adult. Persons with a curve of 10° or less are often thought to have just an asymmetry of the spine, but in children who end up with significant curves, a 10° curve can progress to a 50° curve and a significant deformity if there is enough growing time remaining. Persons with curves measuring under 30° entering adulthood are considered having a mild curve while those over 60° are considered severe. Treatment is recommended, depending on the severity and the age of the person. It would be advantageous to be able to correct the severity before it progresses while the spine is still growing, and various techniques along with various devices, such as the one covered by U.S. Pat. No. 6,554,831 by Rivard et al. have been developed, but these devices and techniques are not well developed and have complications. Adapting a rigid prior art system to the treatment of early onset scoliosis would result in degenerative growth of the spine due to the fixed nature of prior art systems or repetitive surgeries.
There are generally three options to the treatment of scoliosis. The first option is doing nothing. This may be a reasonable decision depending on the age of the patient and the predicted outcome. If the person is a teen or pre-teen and the prediction is that this curve will worsen, then doing nothing may not be appropriate. As the curve progresses, torso deformities occur. In the more severe curves, internal organs are compressed. Without surgery, such patients risk organ damage or failure. On the other hand, if the person has reached maturity, then if the curve is mild, below 40°, it may not increase any more. A second option is to wear a brace. Bracing has been shown to be a somewhat effective method of controlling the curve progression, but it does not cure scoliosis. From a practical aspect though, this treatment is reserved for children and adolescents in whom the prediction of a rapid increase in the curve needs to be thwarted. However, a brace worn even 23 hours per day and worn properly does not guarantee that the curve will not continue to increase.
The third option of treatment is surgery. For those persons who already have a significant curve with a significant deformity, surgery can reduce the curve and significantly reduce the deformity.
The usual scoliosis curve is a thoracic curve. In these curves, the general procedure is a posterior spinal fusion. The fusion is a procedure wherein the individual vertebra are fused to the one above and below. Typically, ten or more segments are included. Scoliosis also affects the lumbar spine as well, often requiring very long fusions of the spine.
It should also be noted that scoliosis a three-dimensional problem, with the curvature of the spine occurring not only in the coronal plane, but usually in angles relative to the coronal plane. One of the aims of surgery is to try to restore the normal contour of the back from both the front view and the side view to restore normal function, balance, and cosmetics.
The spine has normal curves when looking from the side but it should appear straight when looking from the front. Kyphosis is generally a curvature of the upper spine, which when seen from the side the spine is bent forward. Lordosis is a curve that has its convexity anteriorly and concavity posteriorly. People with scoliosis develop additional curves to either side and the vertebrae of the spine twist on each other like a corkscrew.
The present invention addresses various issues encountered in the prior art. Generally, angulation of a polyaxial screw is a means of compensating for a rod that is offset relative to a screw that is inserted into the pedicle, as used above. However, angulation is not the key issue. Rather, the offset is the key issue. With regard to the issue of the rigidity of prior art systems, the present invention allows for locking of a polyaxial screw rigidly at a desired angulation but the present invention also provides load sharing. Hence, the two aspects of the design are not mutually exclusive as in the prior art. Accordingly, combining angulation of the polyaxial screw with additional offset capability allows an increase in the amount of angulation over the prior art. Likewise, combining load sharing of external components into an internal mechanism within the polyaxial screw while still utilizing standard rods eliminates complex external mechanisms or materials subject to failure. Thus, the present invention provides a polyaxial screw that can moderate loads relative to the direction of the load exerted by the force on the rod from adjacent levels.